1. Field of the Invention
The present invention relates to an image forming apparatus using powdered toner.
2. Description of the Prior Art
It is known that an image forming method using an ink-jet printer is employed in word processors, facsimile machines, computers, and the like.
The ink-jet printer is a typical example of a nonimpact printer. In the ink-jet printer, pressure is applied to a prescribed liquid ink while a piezoelectric element or the like applies ultrasonic vibration thereto, so that the ink is discharged into a prescribed electric field from an ink nozzle, the ink droplets being controlled by the electric field and made to adhere to a recording sheet to form an image thereon. Such an ink-jet printing method has the advantage of being able to form a clear image without generating noise during the formation of the image. On the other hand, this method is disadvantageous in that it requires the use of a special kind of recording sheet with its surface appropriately treated, so as to control the speed at which the ink filters into the recording paper. Also, the nozzle through which the ink is discharged tends to become clogged with foreign substances or the like included in the ink.
To overcome the above difficulties with the ink-jet printer, Japanese Laid-Open Patent Publication No. 62-176873 has proposed an image forming method using powdered toner as an image recording medium. This image forming method uses a mesh member to which ultrasonic vibration is applied, and a means for generating electrostatic attraction in accordance with image signals, the mesh member and the electrostatic attraction generating means facing each other. In this method, an insulating recording sheet is placed between the mesh member and the electrostatic attraction generating means, and powdered toner passed through the mesh member is selectively attracted to the recording sheet by means of the magnetic or electrostatic attraction generated according to the image signals, thereby forming a prescribed image on the recording sheet.
In an image forming apparatus utilizing this method, a sheet of plain paper can be used as the recording sheet. Furthermore, since the toner is passed through the mesh member to which ultrasonic vibration is applied, the mesh is prevented from becoming clogged with the toner.
On the other hand, the above image forming method involves the following problems because the insulating recording sheet is placed between the mesh member through which the toner is supplied and the electrostatic attraction generating means.
(1) When the toner is to be applied to the recording sheet by means of the electrostatic attraction, an electric field is formed between the mesh member and the electrostatic attraction generating means. As described above, the insulating recording sheet is placed between the mesh member and the electrostatic attraction generating means, i.e., in the electric field formed therebetween, so that sufficient electric field effect cannot be obtained.
(2) Since the recording sheet is disposed between the mesh member and the electrostatic attraction generating means, the resistivity of the recording sheet affects the density of the image to be formed thereon. The resistivity of the insulating recording sheet varies according to the kind of paper used, so that the density of the resultant image varies from one kind of sheet to another.
(3) Since the recording sheet is disposed between the mesh member and the electrostatic attraction generating means, the water content of the recording sheet also affects the density of the image to be formed thereon. Thus, the density of the resultant image is substantially susceptible to changes in humidity and other environmental conditions which have influence on the water content of the recording sheet. This means that the image density varies from one sheet to another not only when different kinds of recording sheets are used but also when the recording sheets of the same kind are used.
(4) In the case of using electrostatic attraction to apply the toner to the recording sheet, in order to improve the image quality, a large number of electrostatic attraction generating means are required to be disposed in parallel with the recording paper, so that each electrostatic attraction generating means controls the adhesion of the toner to the recording sheet. However, when a large number of electrostatic attraction generating means are disposed in parallel with the recording sheet, the electrostatic attraction generating means are very close to one another. Therefore, the adhesion of the toner cannot be precisely controlled by the respective electrostatic attraction generating means because they are influenced by the electric fields generated by the adjacent electrostatic attraction generating means. On the other hand, if the electrostatic attraction generating means are spaced far apart from the mesh member, it becomes even more difficult to control the adhesion of the toner.
(5) The amount of the toner supplied through the mesh member is not sufficient, which results in an insufficient density of the produced toner image. Also, toner particles, if collected together into lumps by the cohesion of the toner, cannot pass through the mesh member, and this results in an uneven density of the produced image.